Abstract
1160 The aim of this study was to examine the relationship between noninvasive measures of muscle metabolism, derived with magnetic resonance spectroscopy (31P-MRS), and in vitro measures. 8 healthy subjects underwent 31P-MRS measurements and needle biopsies of the medial gastrocnemius. Spectra were acquired using an oblong surface coil at rest, during plantar flexion and recovery. The PCr recovery rate constant (kPCr = 2.07±0.23 min−1) was determined following submaximal exercise (PCr=48.2±4.0 of initial; pH=6.85±0.03) as an index of the in vivo oxidative capacity. In vivo indices of glycolytic potential were derived based on intracellular pH changes during 60 s of maximal rate exercise (PCr=16.8±2.2 of initial; pH=6.58±0.04; dpH/dt=1.38±0.12 pH units/min). Both quantitative histochemistry and whole muscle homogenate biochemical analysis were used to determine in vitro indices of oxidative (SDH, CS, Type I fibers) and glycolytic (GPD, PFK, GAPDH) capacities. Regression analysis between the PCr resynthesis rate constant, CS, SDH activity and fiber type composition demonstrated only a poor to moderate relationship between in vivo and in vitro measures of oxidative metabolism. The highest correlation was found between kPCr and fiber type composition (r=0.59; p=0.06). In contrast, a strong relationship was found between in vitro glycolytic markers and in vivo pH kinetics during maximal exercise. In particular the PFK activity (r=0.92-0.94; p=0.01 − p<0.001) and ratio of SDH-to-GPD (r=−0.86; p=0.03) showed a strong relationship with the level and rate of intracellular acidification during high intensity exercise. In conclusion, our results indicate a strong relationship between in vitro and in vivo measures of glycolysis and a relatively moderate relationship between in vivo and in vitro markers of oxidative phosphorylation. This information is essential if muscle metabolic data is desired in populations where needle biopsies are contraindicated. This work was supported by NIH HD 33738
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